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Spectroscopic properties of Nd:CaF2 crystals with different doping concentrations of Nd3+ and codoping concentrations of Y3+ were investigated. The effect of codoping with Y3+ ions on the absorption and emission spectra, emission lifetimes, quantum efficiency, and emission cross sections of Nd3+ were studied. In particular, codoping with Y3+ ions increases the width of the emission lines over that found with Nd-doped glasses, and allows true CW laser operation around 1.06 μm.

We demonstrate that codoping Nd:CaF 2 single crystals with Y 3 and Lu 3 nonoptically active ions significantly improves their absorption, emission, and fluorescence lifetime properties and their laser performance. The spectral profiles get smoother and wider, wider than that found with standard Nd:glasses; the quantum yields increase; and efficient laser operation is obtained around 1.055 μm.

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Continuous wave laser operation at 1.87 μm of liquid-phase epitaxially (LPE) grown Tm(3+)-doped YLiF(4) (Tm:YLF) layers is demonstrated. The waveguide laser delivers 560 mW by pumping with a Ti:Sapphire laser at 780 nm leading to an efficiency of 76% with respect to the absorbed pump power. This constitutes the first Tm(3+)-doped crystalline fluoride waveguide laser ever demonstrated as well as a record in efficiency and output power for an LPE grown waveguide laser operating in the 2 μm spectral range.

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The existence and the identification of only one or several coparticipating luminescent Yb 3+ centers in the heavily doped Yb:CaF 2 laser crystals which are considered in the development of several high intensity laser chains have been examined first by using two complementary and original experimental approaches, i.e., registration of low temperature site-selective laser excitation spectra related to near-infrared and visible cooperative emission processes, on the one hand, and direct imaging at the atomic scale of isolated ions and clusters using a high-resolution scanning transmission electron microscope in the high angle annular dark-field mode, on the other hand, and then correlating the data with simple crystal field calculations. As a consequence, and although all the experimental details could not be accounted for quantitatively, a good overall correlation was found between the experimental and the theoretical data. The results show that at the investigated dopant concentrations, Yb:CaF 2 should be considered as a multisite system whose luminescent and lasing properties are dominated by a series of Yb 3+ clusters ranging from dimers to tetramers. Hexameric luminescent centers may be dominant at really high dopant concentrations (likely above 20 at. %), as was originally proposed, but certainly not at the intermediate dopant concentrations which are considered for the laser application, i.e., between about 0.5 and 10 at. %.

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